1. Field of the Invention
The present invention relates to bio-separation, and more particularly a portable cartridge for supporting multi-separation columns with integrated detection optics and reagent reservoir and a bio-separation system incorporating the cartridge.
2. Description of Related Art
Bioanalysis, such as DNA analysis, is rapidly making the transition from a purely scientific quest for accuracy to a routine procedure with increased, proven dependability. Medical researchers, pharmacologists, and forensic investigators all use DNA analysis in the pursuit of their tasks. Yet due to the complexity of the equipment that detects and measures DNA samples and the difficulty in preparing the samples, the existing DNA analysis procedures are often time-consuming and expensive. It is therefore desirable to reduce the size, number of parts, and cost of equipment, to ease sample handling during the process, and in general, to have a simplified, low cost, high sensitivity detector.
One type of DNA analysis instrument separates DNA molecules by relying on electrophoresis. Electrophoresis techniques could be used to separate fragments of DNA for genotyping applications, including human identity testing, expression analysis, pathogen detection, mutation detection, and pharmacogenetics studies. The term electrophoresis refers to the movement of a charged molecule under the influence of an electric field. Electrophoresis can be used to separate molecules that have equivalent charge-to-mass ratios but different masses. DNA fragments are one example of such molecules.
There are a variety of commercially available instruments applying electrophoresis to analyze DNA samples. One such type is a multi-lane slab gel electrophoresis instrument, which as the name suggests, uses a slab of gel on which DNA samples are placed. Electric charges are applied across the gel slab, which cause the DNA sample to be separated into DNA fragments of different masses.
Another type of electrophoresis instrument is the capillary electrophoresis (CE) instrument. By applying electrophoresis in a fused silica capillary column carrying a buffer solution, the sample size requirement is significantly smaller and the speed of separation and resolution can be increased multiple times compared to the slab gel-electrophoreses method. These DNA fragments in CE are often detected by directing light through the capillary wall, at the components separating from the sample that has been tagged with a fluorescence material, and detecting the fluorescence emissions induced by the incident light. The intensities of the emission are representative of the concentration, amount and/or size of the components of the sample. In the past, Laser-induced fluorescence (LIF) detection methods had been developed for CE instruments. Fluorescence detection is often the detection method of choice in the fields of genomics and proteomics because of its outstanding sensitivity compared to other detection methods.
Some of the challenges in designing CE-based instruments and CE analysis protocols relates to sample detection techniques. In the case of fluorescence detection, considerable design considerations had been given to, for example, radiation source, optical detection, sensitivity and reliability of the detection, cost and reliability of the structure of the detection optics. In the past, a relatively high power light source is required, such as a Laser. When light is directed through the capillary wall at the separated sample components in the capillary bore, light scatters at the outside capillary wall/air interface and the inside capillary wall/buffer interface (Raman scattering), which obscures or corrupts the fluorescence emission intensity. Similarly, fluorescence emissions scatter at the wall interfaces. In the past, various techniques were developed for more completely collecting the fluorescence emissions to improve signal intensity and hence detection sensitivity. These techniques involve additional moving and non-moving components that add to the relative complexity and cost of the detection setup.
The design limitations of prior art electrophoresis instruments are exacerbated in the development of multi-capillary CE-based instruments. For example, confocal scanning laser induced fluorescence (LIF) detection has been adopted in multi-capillary electrophoresis systems. The scanning confocal detection relies on a scanning optical system. The use of moving parts is not ideal when taking simplicity, robustness, and lower cost of the instrument into consideration. Also, the shallow focal depth of the microscope objective for the confocal detector puts severe demands on the mechanical and optical component tolerances. Further, the optical scanning method generally involves a longer duty cycle per capillary. Thus, should the instrument be scaled up in order to generate higher throughput, the sensitivity of the system may be compromised. Also, another detection method is Sheath Flow detection. The main drawback of the sheath flow detector is the highly sophisticated flow system needed to ensure a reliable sheath flow with minimum optical cross talk between the channels. Extreme demands are put on the optical and mechanical component tolerances in order to meet the robustness demands of end-users. The sensitivity of the device is very good, but it is not obvious that this principle of fluorescence detection is suited for a high-throughput, yet low cost, DNA analysis.
Additional challenges in designing multi-capillary CE-based instruments related to the support of the capillaries. U.S. Pat. No. 5,198,091 to Burolla et al. describes a capillary cartridge for electrophoresis that employs a long length of capillary arrays. This patent may include a hollow space defined about the capillary for circulating coolant fluid but it does not include a reservoir as an integrated part of the cartridge. U.S. Pat. No. 5,413,686 to Klein et al. describes an automated multi-channel capillary electrophoresis analyzer including a plurality of capillaries. Reservoirs are shown in the analyzing apparatus, but they are multiple reservoirs and they are separated from the capillaries, not integrated into a capillary support. Detection optics are also shown in the apparatus, but they are not integrated into a compact capillary support. U.S. Pat. No. 5,338,427 to Shartle et al. describes a single use separation cartridge for a capillary electrophoresis instrument, in which capillary tubes are horizontally disposed in a coplanar array. The single use separation cartridge replaces large reagent reservoirs with hemispherical drops of reagent.
Also, current systems for gel buffer chemistry do not allow use of the CE instrument that is specific with applications. In other words, current CE instruments require matching the capillary (with different coatings and column sizes) with the buffer reagent for different separation applications (different types, speeds, resolutions).